In Greek mythology, Icarus flew too close to the sun, melting his wings, causing him to plunge to his death in the sea below. But if you think his thrilling ascent carried him close to the sun, that was nothing compared to the Parker Solar Probe, which, according to NASA, is the first spacecraft ever to “touch” the sun, a claim so incredible to me that I think it’s worth doing a little probing of our own to decipher what it means.
First off, a bit about the spacecraft: It’s 10 feet by 7 feet by 3 feet and weighs 1,500 pounds. It is the fastest object ever built by humans, capable of traveling at a speed of more than 400,000 mph. At that rate, you could make it from New York to L.A. in 25 seconds, and from Earth to the moon in a little over a half an hour. The solar probe is protected from the sun’s heat by a 4.5-inch-thick carbon shield that can withstand temperatures of 2,500 F.
But wait, we know the sun is way hotter than 2,500 degrees, so how does that work? (I’ll explain in a minute.)
Next, let’s turn our attention to what it means to “touch” the sun. We should be clear here that there is no solid surface or well-defined boundaries to the sun; it’s a burning ball of gas made up almost entirely of hydrogen and helium.
It has seven layers — three inner layers and four outer layers. The innermost and by far the hottest layer of the sun, at 27 million degrees, is called the core. It is surrounded by the radiative zone, which ranges from 3 million to 11 million degrees, and then the convection zone at 3 million to 11,000 degrees. The first of the outermost layers is called the photosphere, which is the visible ball of light we see from Earth. At 11,000 degrees, it is sometimes (though inaccurately) referred to as “the surface” of the sun. Next is the chromosphere (14,000 degrees), followed by the transition region (up to 900,000) and finally the corona, which is the outermost band of the sun’s atmosphere. The corona can reach temperatures of more than 2 million degrees.
When they say the Parker Solar Probe “touched” the sun, they mean it made it into the corona, which extends out about 8 million miles from the photosphere.
Launched in 2018, the Probe entered the corona for the first time in April 2021, and stayed there for five hours. In November, it made it within 5 million miles of the “surface.” In total, it will orbit the sun 24 times, with the deepest penetration coming in 2025 at 3 million miles.
Now, let’s take up the question of how a spacecraft with a heat shield that protects up to 2,500 degrees can withstand temperatures of 2 million degrees.
The answer lies in the distinction between “heat” and “temperature.”
Temperature is a measure of how fast particles are moving; heat is the amount of energy those particles transfer. The corona is a low-density region; the particles are spaced far apart so they don’t transfer much energy to the spacecraft.
I read a great analogy — it’s like the difference between plunging your hand into a boiling pot of water or waving it through a heated oven. You can keep it in the air of the oven a lot longer than you can in the boiling water.
The data gathered from within the sun’s corona will help us further understand weather in space (which sometimes disrupts functions on Earth), the process of nuclear fusion (which could provide keys to safe and sustainable energy) and how solar winds accelerate at super high speeds throughout our solar system.
Should you be tempted to regard the lesson of Icarus as a deterrent to solar exploration, I would point out that it wasn’t the nearness to the sun that was his downfall. It was a lack of discipline, or perhaps an unwillingness to believe “the science.”
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